Retroreflective film or sheeting utilizing microprismatic reflecting elements is used extensively in signing applications, including signing for traffic control. Microprismatic retroreflectors typically comprise a sheet having a generally planar front surface and an array of cube corner elements protruding from the back surface. Cube comer elements comprise interconnected, generally trihedral structures, each of which has approximately mutually perpendicular lateral faces meeting to form a single corner, and thus are characterized as cube-corners. In use, the retroreflector is arranged with the front surface disposed generally toward the anticipated location of both incident light and intended observers. Light incident to the front surface enters the sheet, passes through the body of the sheet and is internally reflected by the faces of the elements so as to exit the front surface in a direction substantially toward the light source. This is referred to as retroreflection. The light rays are typically reflected at the cube faces due either to total internal reflection (TIR) from interfaces with an intentionally entrapped medium of greatly different refractive index, such as air, or to reflective coatings, such as vapor deposited aluminum.
In general, microprismatics reflect light back toward a light source with high efficiency. In addition, microprismatics can spread the retroreflected light into a zone or "cone of light" determined by the particular cube-corner optical design. This enables detection of reflected light at an observation angle other than zero degrees, with zero degrees defined as the vector of perfect retroreflection. The combination of the cube-corner optical geometry and a material of construction having a high index of refraction serves to maximize entrance angularity, which is to say, to maximize the entrance angle, or angle of incidence, up to which good retroreflective performance is observed. Angle of incidence refers to the angle made by the vector of incident light with a vector normal to the planar front surface of the film or sheeting. Polymeric materials of construction are preferred because of their physical properties; thus, within the realm of polymeric materials typically used in these applications, an index of refraction greater than 1.50 is considered high and is desirable. Several cube-corner optical constructions for signing applications include those described in U.S. Pat. Nos. 3,684,348 (Rowland), 4,588,258 (Hoopman), 5,138,488 (Szczech), and 4,775,219 (Appledorn, et al.). U.S. Pat. No. 3,712,706 (Stamm) recognizes that a certain amount of divergence of the retroreflected light from a microprismatic structure is always present due to optical imperfections. In this patent, said divergence due to optical imperfections is minimized, and the arrangement of the optical elements is established such that the angular divergence of the retroreflected light attributable to diffraction is the dominant diverging factor.
U.S. Pat. No. 3,817,596 (Tanaka) seeks to diffuse the retroreflected light by comprising the retroreflector of two types of optical cube-corner elements, the first variety having three reflecting planes positioned such that lines normal thereto intersect each other at right angles, and a second variety having three reflecting planes positioned such that lines normal thereto intersect in a skewed manner while optical axes thereof intersect each other at right angles.
U.S. Pat. No. 4,775,219 (Appledorn, et al.) provides retroreflective articles which may be individually tailored so as to distribute light retroreflected by the articles into a desired pattern or divergence profile. This is accomplished by forming the three lateral faces of the reflecting elements by three intersecting sets of parallel V-shaped grooves, with at least one of the sets including, in a repeating pattern, a groove side angle that differs from another groove side angle of the same set.
PCT Appl. Ser. No. 96/30786 (Nilsen) seeks to redistribute light within the retroreflected cone by texturing a surface of the retroreflective sheeting which is in the path of the light, in order to decrease the high degree of variation within the cone due to the diffraction phenomena discussed in the Stamm patent.
Combining cube-comer optics with materials of construction which further advance the performance of retroreflective articles has become a primary focus of the industry. Polycarbonates and acrylics are optical quality materials commonly utilized in cube-corner retroreflectors, and polybutyrates have also been utilized, as all three provide good optical properties and are easily processed with conventional forming techniques. A variety of replication techniques for manufacturing microreplicated cube-corner materials from thermoplastics have been known to the art. Some of these are detailed in U.S. Pat. Nos. 3,810,804 (Rowland), 4,244,683 (Rowland), 4,332,847 (Rowland), 4,486,363 (Pricone and Heenan), 4,601,861 (Pricone and Roberts), 5,706,132 (Nestegard, et al.), Eur. Pat. Appl. 796,716 (Fujii, et al.), and Eur. Pat. Appl. 818,301 (Fujii, et al.).
Polycarbonate (PC), which has a relatively high isotropic index of refraction of 1.586, has been a preferred material for microprismatics because it more effectively retroreflects to a source which emits light at large angles of incidence to the microprismatic sheeting. As governed by Snell's law, the higher the index of refraction of a material is, the smaller the critical angle (.theta..sub.c) of refraction will be, and thus, the incident angle to which TIR can be achieved within a particular cube-corner element will be larger (FIG. 1). Since less retroreflectivity results as light enters a cube-corner retroreflector at progressively larger angles of incidence, a material which enhances high incidence retroreflectivity, while also enhancing brightness at larger observation angles, would be of particular interest in signing applications. This is particularly true in urban traffic signing applications, where competition from roadway illumination lighting, internally lit signing, automobile headlights, and other light sources may significantly detract from the conspicuity of a retroreflective traffic control device, sign, or the like, and a safety premium is attached to the conspicuity of signage at intermediate distances and wider observational angles. Relatively less activity has been devoted to finding materials which can enhance brightness at larger observation angles than has been the case for finding high-index materials which can improve entrance angularity.
Thus, microprismatic retroreflective materials of the prior art have shortcomings in optical brightness when viewed at wider observational angles or from intermediate distances. Also, the three polymer types from which they are most frequently manufactured are relatively expensive, and are subject to dimensional instability when exposed to moisture. Many other well-known polymeric materials which might otherwise be used in microprismatic retroreflective materials lack sufficient resistance to one or more of the factors involved in "weathering", such as ultraviolet light, heat, moisture, and abrasion. Thus, there is a need in the art for a microprismatic retroreflective material having improved performance at wider observational angles, made from a polymer having an index of refraction at least comparable to that of prior art materials, conventional processability, good weatherability, improved dimensional stability with respect to moisture, and low cost.
These and other needs are met by the present invention, as hereinafter described.